textbox.set focus is not working - why?

Hello Everybody,
I'm trying to set focus to a text box,but i'm getting error message

"Run-time error '5': invalid procedure call or argument"

what i'm trying to do is to select the text in the text box and highlight it ,so if the user will press any key he will see the new key (character).

maybe setfocus is not what i need.

So, if someone know the answer please reply,
thank you all in advance,
ERAN

VB Code:

Public Sub SelectAllText(TheForm As Form)
'rmg
'the purpose of this sub routine is to select all text in a text box on an edit
            TheForm.ActiveControl.SelStart = 0
            TheForm.ActiveControl.SelLength = Len(TheForm.ActiveControl.Text)
End Sub
 
'To use, place this in the Got_Focus event of a Text box:
SelectAllText Me

Dear Hack,
Thank you for your reply,
I used this code of yours from another thread.
Thank you again,
ERAN